Recovery of aromatics by extraction with solvent mixture of n-methyl-pyrrolidone andglycerol



De 10, 1968 KARL-HEINZ ElsENLol-IR ET AL 3,415,742

RECOVERY oF AROMATICS BY EXTRACTION WITH SOLVENT MIXTURE 0FN-METHYL-PYRROLIDONE AND GLYCEROL Filed Dec. 20, 1967 3 Sheets-Sheet 1/77 7 OPA/EVS Dec. 10, 1968 KAR| HE|NZ ElsENLoHR ETAL 3,415,742

RECOVERY OF AROMATICS BY EXTRACTION WITH SOLVENT MIXTURE OFN-METHYL-PYRROLIDONE AND GLYCEROL Filed Deo. 20, 1967 3 Sheeos--SheetI 2De@ 10, 1968 KARL-Hemz ElsENLoHR ET Al. 3,415,742

RECOVERY AROMATICS BY EX CTION WITH SOLVENT MIXTURE N-METHYL-PYRRO ONEAND GLYCEROL Filed Deo. 20, 1967 3 Sheets-Sheet 5 AVVVVA AVAVAXAYAVA AVA@.EV'AYA GLM/@UA 4 I wams/7V cla/wrm 2;

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United States Patent O M 1 Claim. ((11.208-323) ABSTRACT F THEDISCLOSURE Aromatic hydrocarbons are extracted with solvent fromhydrocarbon mixtures containing both aromatic and nonaromatichydrocarbon components using a mixture of N- methyl pyrrolidone andglycerol as the solvent. The extractions are conducted by liquid-liquidextraction.

Cross-reference to related applications This appication is acontinuation in part of copending application Ser. No. 490,818, entitledRecovery of Aromatics by Extraction or Extractive Distillation withSolvent Mixtures, and tiled Sept. 28, 1965, now Patent 3,366,568, in thenames of Karl-Heinz Eisenlohr and Eckart Muller.

Background of the invention Field of the invention-The present inventionprovides an improved process for the recovery of aromatic hydrocarbons,especially benzene, toluene and xylene, from hydrocarbon mixturescontaining the same, by extraction with water free solvent mixturesselective for aromatics which as a whole boil over the boiling pointrange of the aromatics to be recovered. The extraction 'with the solventmixtures concerned is by liquid-liquid extraction.

Description of the prior art- T he recovery of aromatic hydrocarbonsfrom hydrocarbon mixtures by liquid-liquid extraction with selectivesolvents has been practiced on a commercial scale for several decades.However, from the fact that again and again new solvents are suggestedand patented for this purpose, it can be concluded that the idealsolvent has as yet not been found.

In additon to pure solvents, in many instances organic solvent mixtureshave been proposed as the selective solvents. Furthermore, widely spreadpractice is to add certain quantities of water to the solvent. Theaddition of water offers the followng advantages:

(1) The selectivity of the solvent is increased.

(2) The separation, especially of the higher aromatics, from the extractobtained is easily accomplished even with a small diiference between theboiling point of the solvent and the aromatics because the aromaticsdistill azeotropically with the water at temperatures under 100 C.

(3) The sump temperature in the distillation column employed forseparation of the aromatics from the solvent extract is lowered so thatnot only are losses engendered by thermal decompositons reduced but alsothe heat energy required for heating the solvent to the sump temperatureis lowered.

(4) With regard to liquid-liquid extractions a further advantage is thatthe yfield of the existence of two phases `is increased, so that largerquantities of aromatics may be contained in the feed stock withoutleaving the twophase field.

3,415,742 Patented Dec. 10, 1968 ice On the other hand, the followingdisadvantages of the addition of water must be taken into consideration:

(l) The solvent capacity for aromatics is reduced.

(2) The water is vaporized azeotropically with the aromatics to beseparated from the solvent which causes substantial increase in the heatenergy required.

(3) In many instances the water causes undesired chemical reactions,especially hydrolysis..

A number of non-aqueous solvent mixtures have also become known, suchas, for example, mixtures of SO2, ethylene glycol and formamide ormixtures which consist of a primary solvent Iwhich contains glycolderivatives and a secondary solvent such as methanol, ethanol andacetone.

These solvent mixtures, however, have the disadvantage that a portionthereof has a lower boiling point than the aromatics to be recoveredwhereas the other portion has a higher boiling point. The separation ofthe aromatics from the solvent therefor requires substantial plantinvestment as well as high operation costs which in general are notcompensated for by the advantages of the solvent combinations.

Further proposals for solvents include, for example, (l) the combinationof various glycol derivatives, (2) mixtures of two solvents of which thefirst contains l or 2 hydroxyl groups and the second contains 2 or morehydroxyl groups, (3) ethylene carbonate with additions of glycerol,ethylene glycol, pentaerythritol, formamide, formic acid, ethanol amine,monochlorohydrin, acetamid'e, resorcinol or hydroquinone as diluent, (4)alkane dinitriles, dimethyl hydantoin, N-alkylpyrrolidones,-butyrolactone, cyan ethers of diethylene glycol, trior tetraethyleneglycols or any desired mixtures of these solvents which are sel-ectivefor aromatics or olens.

However, without exception, no advantages over the use of a singlesolvent have been proved for these and many other proposals for the useof mixtures of various selective solvents boiling above the aromatics tobe recovered for the liquid-liquid extraction of aromatics, nor have anyindications been given as to what quantity and in what proportions thatvarious components of the mixture should be used in order to attainadvantages over the use of single solvents.

The state of the art concerning the possibilties of use of solventcombinations for the liquid-liquid extraction of aromatics can besummarized in that the effect of the addition of water to a solvent isknown qualitatively, and that something is known concerning theproperties of various combinations of glycol derivatives. However, fromwhich points of view, solvents may otherwise be combined in order thatthey give good properties for liquid-liquid extraction of aromatics hasneither been described in scientific literature -nor in the patentliterature.

Summary of the invention-The object of the present invention is toprovide a process for the recovery of aromatic hydrocarbons fromhydrocarbon mixtures containing the same by means of a liquid-liquidextraction process.

The essence of the present invention lies in the use, as the liquidextracting medium, of a water free solvent mixture which boils above theboiling point range of the aromatics to be recovered and which consistsof a mixture of N-methyl lpyrrolidone and glycerol.

Brief description of the drawings-In the accompanying drawings:

FIG. 1 is a diagram showing the maximum quantity of benzene in the heavyphase (solvent phase) with respect to the composition of the solventmixture N-methyl pyrrolidone (hereinafter referred to as NMP) andglycerol;

FIG. 2 is a diagram showing the dependency of the selectivity of thecomposition of solvent mixture NMP and glycerol;

FIG. 3 is a diagram in which the selectivity is plotted against thepartition-coefficient in dependence on the composition of solventmixture NMP and glycerol;

FIG. 4 is a phase diagram for the 3 component systemNMP-glycerol-benzene;

IFIG. is a diagram showing the dependency of the viscosity at 50 C. ofsolvent mixture NMP and glycerol on its composition; and

FIG. 6 schematically shows an apparatus for carrying out a liquid-liquidextraction according to the invention.

Description of the preferred embodiment-According to the invention, itwas unexpectedly found that despite the large number of solvents knownfor these purposes and despite their very different types of chemicalstructure, certain rules exist, the application of which renders itpossible to select from the boundless large number of possiblecombinations, those which can be expected to have optimal properties.

Above all the following physical properties are desired in a selectivesolvent for the recovery of aromatics by liquid-liquid extraction.

(1) High selectivity.

(2) High capacity.

(3) High aromatics range, that is, the formation of a two-phase systemeven at high aromatic concentrations.

(In the following, the highest aromatics concentration in the 3substance system benzene-n-heptane-solvent at which a two-phase systemis still present and above which separation into two phases no longeroccurs is indicated as the aromatics range).

(4) Low solubility of the solvent in the hydrocarbon phase.

(5) Low viscosity.

(6) High density.

(7) Boiling point above the boiling point range of the aromatics to berecovered.

(8) Thermal and chemical stability at boiling point.

(9) Melting point below ambient temperature.

It was ascertained that these various physical properties do not occurwith complete irregularity in the individual solvents but rather thatcertain conformities to rules can be observed which are of specialsignificance for the selection of a favorable solvent combination.

Of the properties given above, the third, namely, the aromatics range isespecially suited as the classifying principle for dividing the solventinto certain groups. If the solvents are arranged according toincreasing aromatics range they can be divided into three groups:

(A) Solvents with low aromatics range.

(B) Solvents with high aromatics range and (C) Solvents which do nothave unlimited miscibility with aromatics.

A low aromatics range is one of less than 50% by weight and preferablyless than 35% by weight is the maximum of the two-phase region.

The solvents of the rst group A are characterized by their highcapacity, low selectivity and high solubility of the solvent in thehydrocarbon phase. In view of their low selectivity the solvents of thisgroup normally are only employed in combination with water whereby theabove-mentioned disadvantages must be accepted.

It has now been found that most of the solvents of this group possesstwo other important and advantageous properties, namely, low viscosityand good thermal stability at the boiling point. Compounds withheterocyclic ring systems, such as, morpholine, N-methyl pyrrolidone,furfural, keto dioxane, and also dimethyl formamide, aniline, ethylenediamine, nitromethane, ethylene glycol mono-methyl ether, diethyleneglycol mono-methyl ether, and others, all of which have an aromaticsrange below 35%, and diethylene triamine, triethylene tetramne,tetraethylene pentamine, butyrolactone, and dimethyl sulfoxide witharomatics ranges between 35 and 50% especially are illustrative of thegroup of solvents with low aromatics range.

The solvents of group B with a high aromatic range, that is, solventswith an aromatic range of at least 50% in the maximum of the two-phasesystem belong to the second group. They are therefore near thesolubility limit with aromatics, for example, so that they are misciblein all proportions with benzene but not with xylene.

There are solvents of this group which have very high selectivity withgood capacity, the latter, nevertheless, being lower than that of thesolvents of group A. As a consequence, solvents of group B would be theideal solvents for the extraction of aromatics insofar as only theselectivity and capacity is taken into consideration, but remarkablythey without exception possess other physical properties of suchunfavorable values that they must be considered substantialdisadvantages. Above all, representatives of this group all have thedisadvantages that they are no longer stable at their boiling point andhave a high viscosity. In addition, their melting points in someinstances are above normal temperature.

Compounds with double bonded oxygen and dinitriles, such as, forexample, sulfolane, ethylene and propylene carbonate, oxy, thioandimino-dipropionitrile, monomethyl formamide and tetraethylene glycolespecially are illustrative of group B solvents.

The solvents of the third group C have a miscibility gap with aromatics,that means, they are not miscible in all proportions with aromatichydrocarbons. Representatives of this group which dissolve less than 25%of benzene at 20 C. are preferably employed. Their capacity is less thanthat of the solvents of both other groups. Their selectivity is betterthan that of solvents of the first group. Almost all have a highviscosity. Thermal stability at the boiling point is sometimes given.This group mostly contains chain compounds with one or more hydroxylgroups, often also compounds with amino groups, such as, for example,ethylene glycol, diethylene glycol, propylene glycol, glycerol, mono,diand 'triethanol amine, l.4cyclohexane dimethanol, diglycol amine,formamide, malondinitrile, hydrazine and others.

According to the invention it was found that solvent mixtures could beobtained which have substantially better properties than could beexpected from the rule of mixtures by mixing a solvent having a lowaromatic range belonging to the rst group (A) with a solvent having amiscibility gap with aromatics and belonging to the third group (C). Ineach instance the selectivity is better than could be expected from therule of mixtures. The ability to take up aromatics with solvents ofgroup A is limited by their aromatics range, and with solvents of groupC by their low solvent capacity for aromatics. T-he combination of asolvent of group A with a solvent of group C results in la mixturehaving an ability to take up aromatics which is always better than thatof each component and which often exceeds that of the component with thebest take up ability by 50% and more.

An undesirable characteristic of solvents of group C is their highviscosity. In this respect, an unexpected advantage also is found in thecombinations according to the invention as the viscosity of the solventmixture is always less than could be expected from the rule of mixtures.

In addition, it was found that the best mixing solution for the varioussolvent mixtures can quickly be found according to a very definitemethod:

One investigates the three component system, solvent A-solventC-benzene, determines the solubility limits and then the critical point(plait point), that is, the point where at the solubility limit thevolumes of the light and heavier phases are equal. The composition atthis point is the most favorable composition for the liquid-liquidextraction of aromatics.

The critical point for a particular composition of a solvent A andsolvent C, with the prerequisite that solvent A and benzene and solventA and solvent C are completely miscible and that solvent C and benzenehave a miscibility gap, is determined as follows:

Equal parts by volume of benzene and solvent C are placed in a vessel.Two phases are formed, one of which predominantly containsbenzene-generally the lighter phase-in the Ifollowing designated as theHC-phase (hydrocarbon phase) and one which predominantly containssolvent C-generally the heavy phase-designated in t-he following as theS-phase (solvent phase). Then small increments of solvents A are addedwhile observing how both phases behave or change with respect to eachother. When a certain quantity of solvent A has been ladded the phaseseparation line will suddenly disappear and only one phase will bepresent. If by chance the volumes of the phases have been equal justbefore disappearance of the phase separation line the critical pointwould already have been determined. The critical point lies between thecomposition where two phases are last observed and the composition atwhich the phase separation line disappears. Usually, however, thevolu-mes of the phases will be different and shortly beforedisappearance of the phase separation line the phase present in thelarger quantity will be a multiple of the other phase. The procedurewhich follows under such circumstances depends upon which of the twophases was the larger just before disappearance of the phase separationline. If the HC-phase is the smaller one, small increments of benzeneare added to the mixture until the mixture becomes cloudy and againseparates into two phases. If at this point the HC- phase is still thesmaller one, then a mixture of equal parts of benzene and solvent A areadded in small increments until the phase separation line againdisappears so that only one phase is present. Thereafter this procedureis repeated, that is, so much benzene is added until clouding and phaseseparation occurs and then so much of an equal mixture of benzene andsolvent A is added until the phase separation line disappears, at somepoint the condition will occur in which the S-phase rather than theHC-phase is the smaller. At this point the critical point has alreadybeen exceeded and one can usually determine its position byinterpolating between the last and second last measuring point. However,if this is not believed sufciently accurate, because the quantitiesadded were selected too large, several mixtures are prepared which arenear the critical point and which still just separate into two phasesand a mixture of equal quantities of benzene and solvent A is addedthereto in small increments. The mixture in which the two phases are ofequal volume just before the phase separation line disappearscorresponds to the composition of the critical point.

lf, on the other hand, the S-phase is smaller just before disappearanceof the phase separation line during the rst addition of solvent A theprocedure is analogous but the benzene added to cause phase separationis replaced by solvent C and the mixture of equal parts of benzene andsolvent A added to cause disappearance of the phase separation line isreplaced by a mixture of equal parts of solvent C and solvent A and theprocedure repeated until the S-phase just becomes the larger phase andthe critical point interpolated or determined analogously to the aboveprocedures.

When the feed stock from which the aromatics are to be recovered is onein 'which t-he non-aromatics predominantly are parains or if a parafnicantisolvent is used the quantity of solvent C in the mixture can be 5 toless than previously defined. If, on the other hand,

' the non-aromatics in the feed stock contains large quantities of olensand/or naphthenes it often is expedient to increase the quantity ofsolvent C in the mixture about 5 to 10%.

Preferred mixtures of solvents of the group A with solvents of the groupC are as follows:

*2(2-amino ethoxy) ethanol or oxethoxyethylamine.

These relationships are more fully explained in the following withreference to FIGS. 1-6 With the mixture of .N-methyl pyrrolidone(abrreviated as NMP) and glycerol as example. As can be seen from FIG. 1the the ability to take up aromatics of a mixture of a solvent of groupA (NMP) and a solvent of group C (glycerol) is better than that `whichcorresponds to the rule of mixtures and is represented by the brokenline A-C. In general, the ability of a solvent mixture of group A with asolvent of group C to take up aromatics is greater than that of eachsolvent individually. In the illustration given the maximum take up isachieved with a mixture of 41% of glycerol and 59% of NMP and amounts to45 weight percent of benzene in the heavy phase.

The dependency of the selectivity upon the solvent mixture composition,once for 10% benzene and once for 40% benzene in the light phase, isshown in FIG. 2. Both curves run above the tie line between the pointsof the pure solvent compound.

The manner in which the optimum between selectivity and capacity can bedetermined is shown in FIG. 3. In such figure the capacities (partitioncoeicients) and the selectivities for different mixing ratios of NMP andglycerol are respectively plotted as abscissa and ordinates. All dataare based on a 3 component mixture of solventbenzene-heptane at a ratioof 70:20: 10 parts by weight in order to have comparable results. Alogarithmic scale was chosen since the relative change of the twocharacteristics is more important than their absolute change. It can beseen that the points of the individual mixtures lie above and to theright from the line A-C and therefore towards values of higher capacityand higher selectivity. It furthermore can be seen that the range whichextends furthest up and right and therefore the optimal economic rangelies between 35 and 45% glycerol.

This value of about 41% glycerol is also found when the three componentdiagram of both solvents with benzene is investigated and the criticalpoint is determined as described above. FIG. 4 gives the `diagram forthe system NMP-glycerol-benzene. At the critical point P the ratio ofglycerol to NMP is 41:59.

It also can be seen from FIG. 5 that the viscosity at 50 C. of a mixtureof NMP and glycerol, above all in the middle range, is significantlylower than was to be expected from the rule of mixtures.

The present invention is generally suited for the recovery of aromaticsfrom hydrocarbon mixtures containing aromatics by liquid-liquidextraction and also for the recovery of extracts in which theconcentration of aromatics in the hydrocarbon extracts has only beenincreased over that in the starting mixture. An especial advantage ofthe process according to the invention is that it also is adapted forthe Irecovery of high purity aromatics, which in recent times haveassumed considerable significance as starting materials for chemicalsyntheses.

The process according to the invention renders it possible to recoveraromatics whose non-aromatics content is below 0.1% and even under 0.01%without ditliculty. Even higher purity requirements such as absence ofnonaromatics in quantities capable of chromatographic detection can beachieved according to the invention.

The invention is further illustrated by the lfollowing example and FIG.6 which diagramatically shows the apparatus employed in such example.

EXAMPLE 1500 kg./h. of a feed stock of the following composition:

Weight percent B (benzene) 3.3 T (toluene) 20.3 X (Xylene) 28.0 C9aromatics 25.0 Nonaromatic hydrocarbons 23.4

B 5.1 T 5.3 X 6.4 C9 aromatics 4.0 Nonaromatic hydrocarbons 7.2 Solvent72.0

were withdrawn from the :bottom of extractor 2 and supplied to thepreliminary distillation column 4 over conduit 3. Column 4 had 40 actualtrays and was operated with a reux ratio of 0.5 :1. 750 kg./h. of thehead product which consisted of Weight percent B 36 T 4 Nonarornatichydrocarbons 60 was recycled to extractor 2 as reflux over line 5. Thishead product contained all of the non-aromatics originally contained inthe extract. 500 kg./h. of raffinate of the composition Weight percentToluene 1 Xylene 4 C9 `aromatics 25 Nonaromatic hydrocarbons 70 -werewithdrawn from the extractor through line 6.

5500 kg./h. of a non-aromatics free extract were withdrawn from the sumpof column 4 through line 7 and supplied to solvent stripper 8 whichcontained 20 actual trays and was operated with a reflux ratio of 1:1.1000 kg./h. of pure aromatics of the composition Weight percent B 5 T 30X 40 C9 aromatics 25 were taken off overhead through conduit 9. The sumpproduct (4500 kg./h.), which was practically pure solvent mixture, wasrecycled to extractor 2 through line 10.

The aromatics mixture taken off overhead from stripper 8 was distilledin a known manner to separate it into its components. The benzenerecovered had a melting point of 5 .50 C., the toluene recovered had arefractive index nD2=1.4967 and no non-aromatics were discernible gaschromatographically in the xylene fraction.

We claim:

1. A process for separating a hydrocarbon mixture containing aromaticand non-aromatic hydrocarbons into fractions of different degrees ofaromaticity by solvent extraction with a solvent mixture and recoveringa more concentrated aromatics fraction from said mixture which comprisescontacting the vmixture with a water free solvent mixture which as awhole boils above the aromatics to be recovered, said solvent mixtureconsisting of s01- vent (A), N-methyl-pyrrolidone, and solvent (C),glycerol, the ratio of the quantity of said solvent A to the quantity ofsaid solvent C in said solvent mixture being from X:Y to X:Yil0 weightpercent, the ratio X:Y being the ratio of solvents A and C at thecritical point in the three component system solvent A, solvent C andbenzene, said solvent mixture being a mixture of 31 to 51% of glyceroland 69 to 49 weight percent of N-methylpyrrolidone, effecting phaseseparation of the phases thus formed, an extract phase in which thehydrocarbons extracted from said mixture has a higher aromaticconcentration than said mixture and a raffinate phase less aromatic thansaid mixture and distilling an aromatics rich hydrocarbon fraction fromthe solvent mixture in said extract phase.

No references cited.

DELBERT E. GANTZ, Primary Examiner'.

H. LEVINE, Assistant Examiner.

U.S. C1. X.R. 260-674

